How do you sort a stack?

We all would have seen how kids love to stack things and play with it. If we give them lego pieces and ask them to construct a tall building, they will keep stacking them on and on.

Just like how stacking is an interesting gameplay for kids, it is also an interesting data structure concept for programmers.

Stack is a linear type of data structure that follows the LIFO (Last in and first out) principle. With the help of stacks, programmers can easily remove and add more items without any trouble.

Speaking of stacking, how do we sort a stack?

Sorting a stack is usually done to include and maintain a processes context so that the order of the items will remain the same during any interruption.

To make the sorting of a stack possible, we use the recursion method.

Would you like to read more on how the method works on stacks? If yes, dig deep into our article because we’ll give you a detailed guide on this concept.

Head on!

What is a stack?

As the stack follows the LIFO principle, it implies that whichever stack is lastly added, it gets deleted first. In real life, when we remove a stack at the lowest, there are chances of other stacks falling down, right? Similarly, in coding, when we try to remove the least stack, there will be some challenges, but with certain in-built operations, they are avoided. The common operations of stack are:

Push(): It helps push a character to the stack.

Pop(): Aids to pop out the stack’s top character.

Top(): Helps to return the stack’s top element.

Size(): Helps to return the stack’s size.

What is sorting?

Sorting is the process of organizing the data in an increasing or decreasing order based on a linear connection between the information pieces

Names, numerals, and documents can all be sorted. Sorting minimizes the time wasted on searching for something.

For example, when searching for someone's contact, if the contact list is sorted in an alphabetical order, we can easily spot the required person's name with the first letter of their name.

Interesting DSA Fact: Did you know that Richard Bellman found the dynamic programming approach back in the 1950s? Yes, the approach is that old!

Methods to sort a stack

We are sorting a stack to help us change the stack's order without disturbing its size. For example, say we have an input of a stack with elements 1, 2, 3, 4, and 5. Now they’re in the ascending order.

If you want them in descending order, you can sort it and produce the output 5, 4, 3, 2, and 1. To sort a stack, there are two main methods, and they are:

Method 1: Using Temporary Stack

In an array, we can sort it in place and operate comparisons, swap, shifting and other operations with the help of indexes. However, in a stack we can’t easily do it due to the LIFO principle. It clearly implies that if you have to sort a stack, you must modify the last element first.

So does that mean we can’t sort the stack in place? Yes, you can’t sort it in place because only if you pop out certain elements and get some extra place sorting becomes possible.

Now that we need extra space, you might wonder whether it should also be a stack. Yes, indeed, because as we’re going to be using the pop and push operations (stack operations), we can only perform it on a stack.

Hence under this approach, we will create a temporary stack, implement the push and pop operations and finally sort the stack.

Once we create a temporary stack, we will pop out the elements of the current stack and shift it to the temporary one. Once the last element remains, we will shift the temporary stack elements back to the current one. As stacks follow LIFO principle, the result will be automatically in reverse in the temporary stack.

Steps:

Create a tempStack.

Run a loop when the input stack is full.

In every loop iteration, pop out an element, compare it with tempStack top element. In the first loop iteration, we will pop out and send an element to the initially empty tempStack. When it slowly gets full, we start the comparison process.

When the tempStack is complete, it will become the output where the elements are sorted in the required order.

Method 2: Recursion

Recursion is an essential strategy because if we can solve a smaller task, we can surely accomplish the project outcome using the smaller tasks. Recursion is the act of calling oneself. It has a primary case where it tackles the minor problem scenario before calling itself for the small parts.

It uses the notion that when we are in a particular condition, our recursive function will complete processing for smaller solutions. Additionally, we can combine all the solved small tasks to finish the whole problem.

In this method, the idea is similar to the recursive insertion sort. In the insertion sort technique, we will pop the first element from the stack and sort out the remaining elements.

The size of the stack would be n-1. After we sort this stack, we will implement the insertTop function, which will help us add back the top element we initially popped out.

Steps:

Pop out the given stack’s first elements.

Now apply sort to the remaining elements of the stack.

Ensure the size of the stack is n-1.

Now, once the sorting is done, apply the insertTop function.

As the LIFO principle is followed, the top element will be added after the sorted elements.

Interesting DSA Fact: There is a concept called beautiful number. In this concept, when the square of its digitis replaces a number, it becomes 1. Moreover, the recursion approach is used most to solve the beautiful number problem.

Conclusion

Stack is a fascinating concept used as it can be explained with real-life activities. We hope you learned how to sort a stack, stack operations, solution methods, and so on.

If you’re curious about other interesting coding concepts, visit our website and engage in our resources.